The invention relates to a method of processing postal items, which method consists, during a first pass for sorting the items, in taking a digital picture of a postal item bearing address information, in deriving from said picture a digital fingerprint which is a logic identifier for the item, and in storing the fingerprint of the item in a memory in correspondence with data about the item, in particular with data indicative of the address information of the item, and consists, during a second pass for sorting the items, in taking again a digital picture of a current mail item bearing address information, in deriving from said image a digital fingerprint for the current item, and in searching the fingerprints recorded in the first sorting pass for a match with the fingerprint of the current item in order to retrieve the address data of the current item by association.
With such a method, it is unnecessary for every postal item to have an identity code affixed thereto, which code is commonly referred to in the literature as an “ID tag” and can be in the form of a bar code, for example, because a virtual item identifier constituted by the fingerprint of the item is used instead. In practice, the fingerprint is an image signature that is, for example, made up of two components, one of the components characterizing the overall and local distributions of the shades of gray in the image of the item, for example, and the other component characterizing the address information contained in the image of the item at a non-syntactical level of analysis.
However, the use of digital fingerprints raises the problem of searching for a match between a current digital fingerprint and a large number of recorded digital fingerprints, giving rise firstly to difficulty in satisfying real-time requirements, and secondly to a higher probability of confusion in searching for matches. The higher the number of recorded digital fingerprints that are compared with the current fingerprint, the higher the probability of detecting a match between two fingerprints that correspond to two different postal items. Therefore, use of digital fingerprints requires the search for fingerprint matches to be confined to a limited exploration space.
European Patent Document EP 1 222 037 describes a method as defined above that uses fingerprints to identify the items but that, during the second sorting pass, requires the items sorted in the first pass to be fed back into the inlet of the machine in a certain order so that the search for fingerprint matches takes place within a limited exploration space. For that purpose, the sorting outlet bins that contain the items sorted in the first sorting pass must be identified by the machine when the items are fed back in for the second sorting pass, and the fingerprints in the memory must be organized into sequences, each sequence being attributed to a first-pass sorting outlet bin. In practice, such an operating constraint is untenable because, in a sorting process made up of a plurality of sorting passes, it happens frequently that the relative order of the items is changed between the two sorting passes, e.g. consequent upon unloading the sorting outlets into the bins, upon loading the items into the inlet of the machine after a bin has been dropped while the bins are being handled, upon transferring items from one bin to another, or upon the machine become jammed during the second sorting pass.